As radio communication has rapidly spread, lack of frequency bands used for radio communication is posing a problem. An example of the techniques for effectively using frequency bands is beamforming. The beamforming is a technique for enabling radio communication for a specific communication target by emitting directional radio waves, and is a technique that can prevent or reduce interference with other radio systems and the like while maintaining signal quality.
An example of the typical technique for performing the beamforming is a phased array. The phased array is a technique for strengthening a signal propagating in a desired direction by adjusting phases of radio signals supplied to a plurality of antenna elements in a transmitter and combining radio waves emitted from respective antenna elements in space. In order to adjust phases of radio signals supplied to the plurality of antenna elements in the transmitter, it is necessary to perform a calibration for variations in the phase caused by wiring of the substrate or the like, and hence to accurately monitor changes in the phase and changes in the amplitude occurring in the transmitter.
As related techniques, Japanese Unexamined Patent Application Publication No. 2001-330660 and International Patent Publication WO2016/167145 disclose techniques related to the phased array.
Incidentally, an example of the means for monitoring a phase change and an amplitude change occurring in a transmitter of a transceiver using the phased array technique is a technique using loop-back. The technique using the loop-back is a technique for monitoring a phase change or an amplitude change by inputting a part of an output signal of a transmitter to a receiver. In order to accurately monitor the phase change and the amplitude change occurring in the transmitter of the transmitter using the loop-back technique, it is necessary to accurately perform a calibration for a phase difference and an amplitude difference among antenna elements in the receiver of the transceiver using the loop-back technique.
Therefore, it is desired to develop a technique capable of accurately performing a calibration for a phase difference and an amplitude difference among antenna elements in a receiver of a transceiver using the loop-back technique.